Ellipsometry Techniques and Its Advanced Applications …
557
leads to manifold enhancement in their scattering and/or optical absorption crosssections [26, 27]. This phenomenon is known as localized surface plasmon resonance
(LSPR).
The frequency corresponding to resonant interaction (i.e. LSPR frequency) is
greatly affected by the optical and geometrical aspects of metal nanostructures as
well as their surrounding medium viz. size, shape, refractive index, etc. [26, 28, 29].
For instance, LSPR frequency can be tuned from near-infrared to visible regions
by tailoring the size of nanoparticles in the range of 10–100 nm [29]. Likewise,
Plasmonic field strength and LSPR frequency can also be tuned by varying the interparticle gap [30, 31]. According to LSPR phenomenon, there is a resonance frequency
at which enhancement in the near field occurs in the vicinity of nanoparticles (subwavelength) and decays with distance from the nanoparticle [32]. The resonance
frequency of nanoparticles dependent on their size, shape, and interparticle-gap as
well as the type of metals also. In the quasi-static regime, electric field remains
homogeneously distributed over the nanoparticle due to lesser size of nanoparticles
compared to the wavelength of incident light which leads to an excitation of dipoles
only. Thus, 1D or 2D nanoparticles/nanowires become capable of manipulating the
light-matter interaction in terms of plasmonic anisotropy. Optical anisotropy can be
probed by measuring reflection using spectroscopic ellipsometry.
In the following sections, the applications of ellipsometry technique in plasmonics
are discussed in details. For instance, Ar ions having their ion-energy in the range of
250 eV to 500 eV are incident on pristine-Si substrates at an incidence angle in the
range of 65°–67° which lead to the self-organized fabrication of ripple patterns having
amplitude ~2 nm and wavelength of 25–35 nm. Further, these rippled-Si substrates
are used as templates for self-organized deposition of Ag- and Au-nanoparticles,
nanowires, and nanodots at different growth parameters. Therefore, in the upcoming
sections, ripples are referred to a specially-designed self-organized ripple patterns
using low energy ion-beams.
3.2 Reflection Measurements of Nanoparticle Arrays
Reflection measurements are carried out by using linearly s- and p- polarized light
incident on the specimen at oblique angles and reflected light is analyzed using a light
detector where the ratio of reflected and incident light intensities are measured as a
function of wavelength [30, 33]. The schematic diagram for reflection measurement
is presented in Fig. 7. The reflection for p- and s-polarized light is expresses by:
R p =
I r p
I i p
=
E r p
2
E i p
2 , R s =
I rs
I is
=
|E rs |
2
|E is |
2
,
(24)
Reflection measurements are useful in characterizing the position and broadening of LSPR peak. Moreover, in-plane plasmonic anisotropy is also investigated
557
leads to manifold enhancement in their scattering and/or optical absorption crosssections [26, 27]. This phenomenon is known as localized surface plasmon resonance
(LSPR).
The frequency corresponding to resonant interaction (i.e. LSPR frequency) is
greatly affected by the optical and geometrical aspects of metal nanostructures as
well as their surrounding medium viz. size, shape, refractive index, etc. [26, 28, 29].
For instance, LSPR frequency can be tuned from near-infrared to visible regions
by tailoring the size of nanoparticles in the range of 10–100 nm [29]. Likewise,
Plasmonic field strength and LSPR frequency can also be tuned by varying the interparticle gap [30, 31]. According to LSPR phenomenon, there is a resonance frequency
at which enhancement in the near field occurs in the vicinity of nanoparticles (subwavelength) and decays with distance from the nanoparticle [32]. The resonance
frequency of nanoparticles dependent on their size, shape, and interparticle-gap as
well as the type of metals also. In the quasi-static regime, electric field remains
homogeneously distributed over the nanoparticle due to lesser size of nanoparticles
compared to the wavelength of incident light which leads to an excitation of dipoles
only. Thus, 1D or 2D nanoparticles/nanowires become capable of manipulating the
light-matter interaction in terms of plasmonic anisotropy. Optical anisotropy can be
probed by measuring reflection using spectroscopic ellipsometry.
In the following sections, the applications of ellipsometry technique in plasmonics
are discussed in details. For instance, Ar ions having their ion-energy in the range of
250 eV to 500 eV are incident on pristine-Si substrates at an incidence angle in the
range of 65°–67° which lead to the self-organized fabrication of ripple patterns having
amplitude ~2 nm and wavelength of 25–35 nm. Further, these rippled-Si substrates
are used as templates for self-organized deposition of Ag- and Au-nanoparticles,
nanowires, and nanodots at different growth parameters. Therefore, in the upcoming
sections, ripples are referred to a specially-designed self-organized ripple patterns
using low energy ion-beams.
3.2 Reflection Measurements of Nanoparticle Arrays
Reflection measurements are carried out by using linearly s- and p- polarized light
incident on the specimen at oblique angles and reflected light is analyzed using a light
detector where the ratio of reflected and incident light intensities are measured as a
function of wavelength [30, 33]. The schematic diagram for reflection measurement
is presented in Fig. 7. The reflection for p- and s-polarized light is expresses by:
R p =
I r p
I i p
=
E r p
2
E i p
2 , R s =
I rs
I is
=
|E rs |
2
|E is |
2
,
(24)
Reflection measurements are useful in characterizing the position and broadening of LSPR peak. Moreover, in-plane plasmonic anisotropy is also investigated